Light emitting devices with compact active regions

ABSTRACT

A light emitting device includes a region of first conductivity type, a region of second conductivity type, an active region, and an electrode. The active region is disposed between the region of first conductivity type and the region of second conductivity type and the region of second conductivity type is disposed between the active region and the electrode. The active region has a total thickness less than or equal to about 0.25λ n  and has a portion located between about 0.6λ n  and 0.75λ n  from the electrode, where λ n  is the wavelength of light emitted by the active region in the region of second conductivity type. In some embodiments, the active region includes a plurality of clusters, with a portion of a first cluster located between about 0.6λ n  and 0.75λ n  from the electrode and a portion of a second cluster located between about 1.2λ n  and 1.35λ n  from the electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.10/632,720, filed Jul. 31, 2003, which claims the benefit of U.S.Provisional Application 60/435,838, filed Dec. 20, 2002. Both 10/632,720and 60/435,838 are incorporated herein by this reference.

BACKGROUND

1. Field of Invention

The invention is related to light emitting devices with reflectivecontacts.

2. Description of Related Art

Semiconductor light emitting devices such as light emitting diodes(LEDs) are among the most efficient light sources currently available.Material systems currently of interest in the manufacture of highbrightness LEDs capable of operation across the visible spectrum includegroup III-V semiconductors, particularly binary, ternary, and quaternaryalloys of gallium, aluminum, indium, and nitrogen, also referred to asIII-nitride materials; and binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and phosphorus, also referred to asIII-phosphide materials. Often III-nitride devices are epitaxially grownon sapphire, silicon carbide, or III-nitride substrates andIII-phosphide devices are epitaxially grown on gallium arsenide by metalorganic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE)or other epitaxial techniques. Often, an n-type layer (or layers) isdeposited on the substrate, then an active region is deposited on then-type layers, then a p-type layer (or layers) is deposited on theactive region. The order of the layers may be reversed such that thep-type layers are adjacent to the substrate. Needed in the art are LEDstructures that increase the amount of light extracted from the device.

SUMMARY

In accordance with embodiments of the invention, a light emitting deviceincludes a region of first conductivity type, a region of secondconductivity type, an active region, and an electrode. The active regionis disposed between the region of first conductivity type and the regionof second conductivity type and the region of second conductivity typeis disposed between the active region and the electrode. The activeregion has a total thickness less than or equal to about 0.25λ_(n) andhas a portion located between about 0.6λ_(n) and 0.75λ_(n) from theelectrode, λ_(n)=λ_(vacuum)/n, where n is the index of refraction in theregion of second conductivity type.

In some embodiments, the active region includes a plurality of clusters,with a portion of a first cluster located between about 0.6λ_(n) and0.75λ_(n) from the electrode and a portion of a second cluster locatedbetween about 1.2λ_(n) and 1.35λ_(n) from the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a light emitting device according to embodiments ofthe present invention.

FIG. 2 illustrates a portion of the light emitting device of FIG. 1covered with an encapsulating gel.

FIG. 3 illustrates the angular distribution of flux of light emittedfrom III-nitride single quantum well silver/nickel contact lightemitting devices having different amounts of material separating theactive region from the reflective contact. The devices are fabricated onsapphire substrates and are packaged with a silicone encapsulating gel.The amount of material separating the active region from the reflectivecontact is expressed in terms of wavelength, λ_(n).

FIG. 4 illustrates extraction efficiency of top light as a function ofseparation between the active region and reflective contact for severaldevices.

FIG. 5 illustrates a method of determining the separation between theactive region and reflective contact of a light emitting device.

FIG. 6A illustrates one embodiment of active region 1 of FIG. 1. FIG. 6Billustrates the location of the device layers illustrated in FIG. 6A.

FIG. 7A illustrates an alternative embodiment of active region 1 ofFIG. 1. FIG. 7B illustrates the location of the device layersillustrated in FIG. 7A.

FIG. 8 illustrates an alternative embodiment of a light emitting device.

FIG. 9 is an exploded view of a packaged light emitting device.

FIG. 10 illustrates a thin film electroluminescent device according toembodiments of the present invention.

DETAILED DESCRIPTION

The examples described below are directed to semiconductor lightemitting devices. Embodiments of the invention may be applicable toorganic light emitting devices, or any other suitable flip chip device.

FIG. 1 illustrates a light emitting device according to embodiments ofthe present invention. A group of semiconductor layers including ann-type region 3, a light emitting active region 1, and a p-type region 5are formed over a substrate 2. The semiconductor layers may be, forexample, III-nitride layers, III-phosphide layers, II-VI layers, or anyother suitable material. Each of n-type region 3, active region 1, andp-type region 5 may include multiple layers of the same or differentcomposition, thickness, and dopant concentration. A portion of p-typeregion 5 and active region 1 are removed to expose a portion of n-typeregion 3. An n-electrode 10 is deposited on n-type region 3 and ap-electrode 4 is deposited on p-type region 5. At least one of the p-and n-electrodes are highly reflective of light emitted by active region1. The device is physically mounted on and electrically connected to asubmount 9 by interconnects 8.

FIG. 8 illustrates an alternative embodiment of a semiconductor lightemitting device. In the device of FIG. 8, p-type region 5 is separatedfrom p-electrode 4 by a reflective surface 50 such as a distributedBragg reflector (DBR).

FIG. 10 illustrates a thin film luminescent device such as an organiclight emitting diode, according to embodiments of the present invention.A phosphor light emitting layer 304 is sandwiched between two insulators303 and 305. Contact is made to the device through metal layer 306 andtransparent electrode 302. Light is extracted through a glasssuperstrate 301. Organic light emitting diodes are described in moredetail in Kristiaan Neyts, “Microcavities for ElectroluminescentDevices,” Chapter 4, Electroluminescence II, ed. Gerd Mueller,Semiconductors and Semimetals, Vol. 65.

Light extraction efficiency may be improved by controlling the placementof the light emitting layers relative to reflective layers in thedevice. In the device of FIG. 1, the placement of active region 1 iscontrolled relative to the highly reflective p-electrode 4. In thedevice of FIG. 8, the placement of active region 1 is controlledrelative to DBR 50. In the device of FIG. 10, the placement of phosphorlayer 304 is controlled relative to metal layer 306.

Returning to the device of FIG. 1, assuming the p-electrode is aperfectly conducting metal, when the center of the active region isbrought within approximately an odd multiple of quarter-wavelengths oflight within the material ((2 i+1)λ_(n)/4, where i=0, 1, 2, . . . ) fromthe reflective p-electrode, constructive interference of the downwardand upward traveling light results in a radiation pattern that emitspower preferentially into the escape cone (ƒ˜0° as shown in FIG. 1).This enhancement is in a direction close to normal to the semiconductorlayers/substrate and is not susceptible to total internal reflectionback into the semiconductor layers. Alternatively, slight detuning ofthe resonance condition, by moving the active region slightly closer to(or farther from) the p-electrode reflector, may be preferred tooptimize the emission of light into the escape cone, and thus the totaltop surface extraction from the chip. For maximum efficiency in mostapplications, the distance between the active region and a perfectlyconducting metal p-electrode should be approximately onequarter-wavelength.

Further retuning of the resonance condition for maximum extraction in adevice with a nonideal metal contact depends on the phase shift of lightreflecting off the metal. Methods for determining the phase shift of anactual reflective contact, then determining the optimal placement of anactive region relative to that contact based on the phase shift aredescribed below. Though the below description often uses the example ofa III-nitride device formed on a sapphire substrate, it will be clear toa person of skill in the art that the methods described are readilyapplicable to other materials systems, other contact metals, and othergrowth substrates.

The total amount of light emitted from the LED (i.e., the totalintegrated flux) is the integrated flux emitted from the topside(towards the substrate) of the device added to the integrated fluxemitted from the sides of the device. Side-emitted light is typicallyguided to the sides of the device by a waveguide created by reflectivesurfaces and various device layers having different indices ofrefraction. Waveguided light typically undergoes several reflections onits path to the side of the device, losing intensity with eachreflection. In addition, light passing through the active region may beabsorbed. Thus it is advantageous to extract as much light as possiblefrom the topside of the device in the first pass, tending thereby toreduce internal losses and increase the total integrated flux.

Flipchip LEDs have a “top escape cone” near the active region such thatlight impinging on the topside from within the LED and lying within theescape cone exit directly from the topside of the device. For economy oflanguage, we refer to the top escape cone merely as the “escape cone,”understanding thereby that maximum topside light emission is asignificant LED performance goal. The escape cone is determined byseveral device parameters including the indices of refraction of thevarious layers within the device, according to Snell's law. Light beamsimpinging on the topside outside the escape cone undergo total internalreflection. Such internally reflected light typically exits from theside of the device or undergoes further internal reflections and loss ofintensity within the device. Thus, one approach to increasing theintensity emerging from the topside of the LED is to increase the fluximpinging on the topside that lies within the escape cone.

FIG. 2 depicts light escaping from a portion of the device illustratedin FIG. 1. The reflective positive ohmic contact 4 lies at a separationd from active region 1 and has p-type region 5 lying between the activeregion 1 and the contact 4. Region 5 can comprise one layer or cancomprise multiple sublayers having distinct compositions, dopingcharacteristics and refractive indices from sublayer to sublayer or agradation of compositions, electrical properties and optical propertiesthroughout the thickness of p-type region 5.

To be clear in our descriptions, we consider in detail the case of aIII-nitride device with Al_(x)In_(y)Ga_(z)N layers, wherein 0≦x≦1,0≦y≦1, 0≦z≦1, and x+y+z=1, including a GaN base layer 3 and a singlep-type layer 5, each of base layer 3 and p-type layer 5 having asubstantially uniform index of refraction throughout. Generalizations tolayers having non-uniform indices of refraction (such as arising frommultiple layers of different materials, gradations of opticalproperties, and the like) is straight-forward by the use of opticaldistances obtained by summing or integrating (physical thickness oflayer i)/(index of refraction of layer i) over various layers of baselayer material. Therefore, examples presented herein for layers having auniform index of refraction are illustrative and not limiting.

Light generated in active region 1 and emerging from the topside of theLED passes through n-type region 3, substrate 2 and encapsulating gel 7and undergoes refraction at each, as depicted by beam 8 in FIG. 2. GaNbase layer 3 has an index of refraction n₁=2.4. Sapphire substrate 2 hasn₂=1.8 and a typical encapsulating gel 7 has n_(3=1.5). Thus, refractionaway from the normal occurs as depicted in FIG. 2, causing light 8 toemerge from substrate 2 into encapsulating gel 7 with an angle from thenormal of θ₃.

As light travels from the site of its formation into the encapsulatinggel 7 through successive regions of lower index of refraction (3, 2 and7), the possibility of total internal reflection arises at eachinterface. That is, if beam 8 strikes the 3-2 or 2-7 interface from thehigher index side at too glancing an angle, no light will enterencapsulating gel 7.

Applying Snell's Law to FIG. 2 gives n₁ sin θ₁=n₂ sin θ₂=n₃ sin θ₃. Theescape cone is determined by θ₃=90°, or sin θ₁(escape)=(n₃/n₁). Usingthe above values for the indices of refraction for GaN, sapphire andencapsulating gel yields θ₁(escape)≈38.7°. Thus, light striking then₁-n₂ interface from the n₂ side will not emerge from the topside of thedevice if the angle of incidence exceeds θ₁(escape), about 38.7°.

Light emitted from electron-hole recombination occurring in the activeregion 1 can be directed into the transparent substrate directly, suchas beam 6 d, or following reflection from ohmic contact 4 such as beam 6r. The coherence length for light emitted in active region 1 istypically around 3 μm in GaN. Thus, if separation d is less than about50% of the coherence length (d

1.5 μm in GaN), strong interference between direct (6 d) and reflected(6 r) beams is expected to occur. The interference pattern is influencedby the distance between active region 1 and reflective contact 4.

The reflected light 6 r suffers a loss in intensity and a phase shift asit bounces off reflective contact 4. To increase the light output from aflip chip and to reduce intensity loss to contact 4, contact 4 may bechosen to have high reflectivity. For example, contact 4 may have areflectivity greater than 50%, usually has a reflectivity greater than80%, and, in some embodiments, preferably has a reflectivity greaterthan 90%. The phase shift of the reflected light 6 r depends on the nand k values of the metals or metal alloys used in reflective contact 4,and therefore, the phase shift will change depending on the metal type.This phase shift also influences the interference pattern. For a perfectconducting metal, the phase shift will be 180°. Generally, the contactson real devices are not perfect conducting metals.

Once the phase shift of light reflected from reflective contact 4 isdetermined, the interference pattern of light escaping the device can becalculated as a function of the distance between active region 1 andreflective contact 4. As described above, the critical angle for lightescaping from a GaN/sapphire/encapsulating gel interface is 39 degrees,thus only the portion of light in GaN directed within this 39 degreescan escape through the topside of substrate 2. One way to increase lightextraction from the light emitting device is to concentrate lightintensity within the escape cone. Since the interference patternsemerging from the active region are controlled by the distance betweenactive region 1 and reflective contact 4 for a given contact material,the light intensity within the escape cone may be maximized byappropriately selecting the distance between active region 1 andreflective contact 4.

The electric field of the directly emitted light 6 d from a singlequantum well active region is given by:{right arrow over (E)} ₀ =w ₀*exp(−i{right arrow over (k)}*{right arrowover (x)})  (1)The electric field of the reflected light 6 r from a single quantum wellactive region is given by:{right arrow over (E)} _(R) =w _(R)*exp(−i{right arrow over (k)}*{rightarrow over (x)}Φ+Φ′))  (2)where w₀ is the amplitude of emitted light 6 d, w _(r) is the amplitudeof reflected light 6 r, k is the direction vector, x is the positionvector, Φ is the phase shift upon reflection off reflective contact 4,and Φ′ is the phase shift due to the optical path length differences.

The intensity in the device, as a function of angle, θ, is then givenby:|{right arrow over (E)} _(Total)|²=({right arrow over (E)} ₀ +{rightarrow over (E)} _(R))*({right arrow over (E)} ₀ +{right arrow over (E)}_(R))=w ₀ ² +w _(R) ²+2w ₀ w _(R) cos(Φ+Φ′).  (3)

The phase shift due to optical path length difference Φ′ depends on thedistance d between the single quantum well active region 1 andreflective contact 4, the angle θ, the wavelength λ, and n (the index ofrefraction of the device layers, GaN in the above example) according toequation (4), where λ_(n)=λ/n: $\begin{matrix}{\Phi^{\prime} = {{2\pi\frac{\Delta\quad x}{\lambda_{n}}} = {2\pi{\frac{2d*\cos\quad\theta}{\lambda_{n}}.}}}} & (4)\end{matrix}$

The phase shift due to reflection from reflective contact 4, Φ, can becalculated from the n and k values of the metal in reflective contact 4if the metal is known. See, for example, Max Born & Emil Wolf,Principles of Optics, p. 628-630 (1980), which is incorporated herein byreference. If the n and k values of reflective contact 4 are not known,the phase shift Φ may be determined by, for example, the followingmethod. A detector is positioned to collect light that is emittedthrough the substrate normal to a device under test, in a smallcollection cone (θ˜6°). A series of devices having a varying distancebetween active region 1 and reflective contact 4 are fabricatedincluding the material with unknown phase shift as reflective contact 4.If the wavelength of emitted light is constant, the extractionefficiency of the series of devices will only vary as a function of thedistance d between active region 1 and reflective contact 4.

According to equation 3, the minimum in the extraction efficiency islocated where Φ+Φ′=m*π (m is an odd integer), where Φ′ is the phasedifference due to the optical path differences between directly emittinglight 6 d and reflected light 6 r. With a series of devices each havinga single quantum well placed at a different distance from reflectivecontact 4, the external quantum efficiency of reflective contact 4 maybe measured by the detector as a function of distance between the singlequantum well active region 1 and reflective contact 4. Using a secondreflective contact 4 with a known phase shift, another curve of externalquantum efficiency versus distance can be measured or calculated for asecond set of devices with the same active-region-to-reflective-contactdistances. External quantum efficiency is the product of internalquantum efficiency and extraction efficiency, EQE=C_(ext)*IQE, thus toeliminate the wafer to wafer differences in the internal quantumefficiency and to obtain the ratio of the extraction efficiencies, thepoints on the first curve can be divided by the points on the secondcurve, such that:EQE _(unknown) /EQE _(known) =C _(ext,unknown) /C _(ext,known).  (5)

Equation (3) may be substituted for C_(ext,unknown) and C_(ext,known),the measured values of external quantum efficiency for both devices maybe substituted for EQE_(unknown) and EQE_(known), then Equation (5) maybe solved for Φ_(unknown metal). Since d, θ, λ, m, Φ_(known metal), andreflectivity are known, Φ_(unknown metal) can be calculated for anyunknown reflective contact 4. Other methods may be used to determine Φ.See, for example, P. Maaskant et al., Fabrication of GaN-based ResonantCavity LEDs, PHYS. STAT. SOL. (submitted Feb. 19, 2002).

Once the phase shift due to reflection (1D is known, the intensity canbe calculated as a function of distance d and angle θ using equation 3above. FIG. 3 depicts computer-generated examples of the top-sidefar-field emitted light intensity (or flux) as a function of thedirection of emission into silicone with respect to the normal to theLED, θ₃, defined in FIG. 2. The curves in FIG. 3 are based on a singlequantum well III-nitride device fabricated on a sapphire substrate withsilicone as encapsulating gel 7. Various values of d are depicted, fromcurve a of FIG. 3 having d=0.5λ_(n), to curve i of FIG. 3 havingd=1.3λ_(n), where λ_(n) is the wavelength of the light in thesemiconductor material separating the active region and the reflectivecontact. The units of flux are arbitrary as only the variations of fluxwith angle are of concern. The radiation patterns depend upon thedistance, d, the wavelength of emitted light, and the effective indicesof refraction of the materials through which the light passes in exitingfrom the LED, among other factors. The radiation patterns clearly changeas d changes, changing the flux lying within the escape cone of 38.7°.

As illustrated in FIG. 3, a maximum in total emitted flux may occur fora radiation pattern not peaked about the central perpendicular axis ofthe light emitting region. That is, spacing the reflective plane fromthe light emitting region such that flux intensity is directed primarilynormal to the surface (0 deg. in FIG. 3 or “on-axis”) does notnecessarily lead to maximum total emitted flux. Curve “f” in FIG. 3provides marked on-axis peaking of emitted radiation, but at aconsiderable sacrifice in the total emitted flux. Thus, spacing thelight emitting region from the reflector so as to maximize on-axis lightemission intensity may be suboptimal for obtaining maximal LED totalflux.

The curves in FIG. 3 may be integrated and added to light emitted fromthe side of the device to generate curve a of FIG. 4. FIG. 4illustrates, for four devices, computer generated plots of extractionefficiency of total flux of a 1 mm×1 mm flip chip as a function ofdistance between active region 1 and reflective contact 4 divided byλ_(n). Curve a in FIG. 4 represents the results for a III-nitride singlequantum well device with a sapphire substrate, a silicone encapsulatinggel, and a reflective contact having silver/nickel contact. FIG. 4illustrates the second and third maxima. The second maximum inextraction efficiency occurs between about 0.6λ_(n) and about 0.75λ_(n),and the third maximum in extraction efficiency occurs between about1.2λ_(n) and about 1.35λ_(n). Thus, for maximum extraction efficiency ina III-nitride device with a (thin Ni)/Ag contact, the center of a singlequantum well active region 1 should be placed between about 0.5λ_(n) andabout 0.8λ_(n), or between about 1.1λ_(n) and about 1.4λ_(n) fromreflective contact 4.

The location of the maxima in a curve of extraction efficiency vs.distance may shift as the wavelength of the light changes. Thewavelength of the light affects the phase shift due to reflection fromthe metal, Φ. If Φ is calculated from known n and k values, thecalculation takes into account the wavelength. If Φ is measured asdescribed above, Φ must be measured for a particular wavelength toensure proper selection of the distance between the active region andthe reflective contact. The table below lists spacing ranges d between asingle III-nitride quantum well and a Ni/Ag contact corresponding to thesecond, third, and fourth maxima in a plot of extraction efficiency vs.d for three different wavelength devices. 450 nm device 505 nm device530 nm device n = 2.4 d/λ_(n) d/λ_(n) d/λ_(n) Optimal 0.5-0.8 0.53-0.830.55-0.85 Preferred 0.6-0.7 0.63-0.73 0.65-0.75 Optimal 1.05-1.351.08-1.38 1.1-1.4 Preferred 1.15-1.25 1.18-1.28 1.2-1.3 Optimal 1.6-1.91.63-1.93 1.65-1.95 Preferred 1.65-1.75 1.68-1.78 1.7-1.8A first maximum generally lies between about 0.1λ_(n) and about0.3λ_(n).

Though the above example is directed to a III-nitride device with anickel/silver contact, a silicone encapsulating gel, and a sapphiresubstrate, it will be apparent to a person of skill in the art that themethod of choosing the spacing between the active region and reflectivecontact can be applied to other materials systems including but notlimited to III-phosphide, III-arsenide, and II-VI, to other contactmaterials, to other encapsulating materials, and to other substrates.For example, a 450 nm III-nitride device with a pure silver contact hasa second maximum in extraction efficiency between about 0.65λ_(n) andabout 0.75λ_(n), a third maximum in extraction efficiency between about1.15λ_(n) and about 1.38λ_(n), and a fourth maximum in extractionefficiency between about 1.73λ_(n), and about 1.85λ_(n). A 625 nmIII-phosphide device (n=3.4) with a gold contact has a first maximum inextraction efficiency between about 0.1λ_(n) and about 0.3λ_(n), asecond maximum in extraction efficiency between about 0.6λ_(n), andabout 0.75λ_(n), a third maximum in extraction efficiency between about1.1λ_(n) and about 1.25λ_(n), a fourth maximum in extraction efficiencybetween about 1.6λ_(n) and about 1.8λ_(n), and a fifth maximum inextraction efficiency between about 2.18λ_(n) and about 2.28λ_(n).

FIG. 5 illustrates a method for determining the spacing between theactive region and reflective contact. First, in stage 202, the escapecone is calculated using the index of refraction of the semiconductormaterial, the substrate, and the encapsulation material or air, if noencapsulation material is used. In stage 204, the phase shift due tooptical path length and phase shift due to reflection are thencalculated or measured, as described above. The phase shift due tooptical path length and phase shift due to reflection depend on thematerial used in the reflective contact and the wavelength of the light.In stage 206, the radiation pattern is calculated for various spacingsbetween the active region and reflective contact using the phase shiftsdetermined in stage 204, yielding a graph such as FIG. 3. The radiationpattern depends on the wavelength of light. In stage 208, the extractionefficiency is calculated as a function of the distance d between theactive region and the reflective contact. The extraction efficiencydepends on the radiation pattern calculated in stage 206 and the escapecone calculated in stage 202. An example of extraction efficiency as afunction of d is illustrated in FIG. 4. The maxima in the plot ofextraction efficiency as a function of d are identified in stage 210,which determine the spacing between the active region and the reflectivecontact which outputs the most light. The method illustrated in FIG. 5is described in more detail in U.S. application Ser. No. 10/158,360,titled “Selective Placement Of Quantum Wells In Flipchip Light EmittingDiodes For Improved Light Extraction,” filed May 29, 2002, andincorporated herein by reference.

The method described in FIG. 5 may be used to determine the separationbetween the reflective contact and active region of a III-phosphidedevice, where the device layers are Al_(x)In_(y)Ga_(z)P, wherein 0≦x≦1,0≦y≦1, 0≦z≦1, and x+y+z=1. III-phosphide devices often have an AlInPlayer adjacent to the active region, and a GaInP contact layer adjacentto the reflective contact. The reflective contact is selected for ohmiccontact with GaInP. Examples of suitable reflective contact materialsare Al, Ag, and Au. The reflective contact metal may be alloyed to theGaInP, which may reduce the reflectivity of the contact. To alleviate atrade-off between ohmic properties and reflective properties, a two-partcontact may be used. To form a two part contact, first a layer ofsuitable ohmic metal is deposited and alloyed to the GaInP. Then, themetal is etched into a pattern of fine lines whereby most of the surfacearea of the GaInP is revealed, the fine lines serving to conduct currentinto the device. The exposed areas of the GaInP are then chemicallyetched away revealing the AlInP cladding layer beneath, therebyeliminating a large part of the adsorbing GaInP from the optical path.Finally, a suitable reflector, chosen for its optical properties withoutregard to the resistivity of the contact it makes to the AlInP, isdeposited over both the fine lines of the first metal and the exposedAlInP. In this manner, current is conducted into the device by the firstmetal, and a highly reflective second metal serves as a mirror.

Curve a of FIG. 4A presents data for a single quantum well activeregion. However, the methods described herein are not limited to singlequantum well devices and can also be used in connection with multiplequantum well (MQW) active regions. For example, the center of brightnessand/or physical center of MQW active regions may be placed at aseparation corresponding to a maximum on the appropriate plot ofextraction efficiency of top-side flux as a function of distance betweenthe active region and the reflective contact. Such an embodiment isillustrated in FIGS. 6A and 6B. The active region illustrated in FIG. 6Ahas two quantum well layers 15 separated by a barrier layer 17. Asillustrated in FIG. 6B, the center of barrier layer 17, which is thecenter of active region 1, is located at the first peak of curve b ofFIG. 4, the curve corresponding to a two quantum well active region. Thefirst peak of curve b corresponds to the second maximum in a plot ofextraction efficiency vs. distance between the center of the activeregion and the p-electrode, and is located at a distance of about0.67λ_(n).

The extraction efficiency of a device can be improved by designing theactive region and the layers between the active region and thereflective electrode such that each of the quantum wells are located asclose as possible to a maximum on a plot of extraction efficiency vs.distance from the reflective electrode. FIGS. 6A, 6B, 7A, and 7Billustrate examples of devices so designed.

The device illustrated in FIGS. 6A and 6B may be, for example, aIII-nitride device emitting light at a wavelength ranging from UVthrough green. Each of quantum wells 15 may have a thickness rangingfrom about 10 angstroms to about 60 angstroms, usually has a thicknessbetween about 15 angstroms to about 40 angstroms, and preferably has athickness of about 30 angstroms. The composition of quantum wells 15depends on the color of light to be emitted by the device. Each ofquantum wells 15 need not have the same thickness and composition.Barrier layer 17 may have a thickness ranging from about 50 angstroms toabout 200 angstroms, and usually has a thickness of about 85 angstroms.A barrier layer in a shorter wavelength device may be thinner than abarrier layer in a longer wavelength device. P-type region may contain,listed in order from the active region to the p-electrode, an AlGaNconfining layer 5 a, a first GaN layer 5 b, and a second GaN contactlayer 5 c. In some embodiments, confining layer 5 a may have a thicknessbetween 100 angstroms and 1000 angstroms, and usually has a thicknessbetween about 100 angstroms to about 400 angstroms; first GaN layer 5 bmay have a thickness between about 100 angstroms and about 1000angstroms, and usually has a thickness between about 400 angstroms andabout 900 angstroms; and second GaN contact layer 5 c may have athickness between about 50 angstroms and about 500 angstroms, andusually has a thickness between about 50 angstroms and about 250angstroms. The reflective p-electrode in the device illustrated in FIGS.6A and 6B may be, for example, a multi-layer electrode with a thin layerof Ni sandwiched between GaN contact layer 5 c and a thick layer of Ag.

In the embodiment illustrated in FIGS. 6A and 6B, the entire activeregion is located surrounding a peak on a curve of extraction efficiencyvs. distance between the active region and the p-electrode. Though theactive region is placed at a distance corresponding to the secondmaximum on the curve of extraction efficiency vs. distance in FIG. 6B,the placement of the active region may correspond to the third or ahigher local maximum. Usually the active region is not placed near thefirst maximum, since placement of the active region at the first maximummay result in a p-type region that is too thin. Usually the fourth andhigher maxima are not used as such placement may result in a p-typeregion that is too thick, and a decrease in extraction efficiency. Thep-type region is generally fabricated at a higher temperature than thequantum wells, thus the fabrication of a thick p-type region may resultin fabrication conditions that damage the quantum wells. In addition,the extraction efficiency beyond the fourth local maximum issignificantly lower than for the second, third, and fourth local maxima.

Though two quantum wells are illustrated in FIG. 6A, more or fewerquantum wells may be used provided the portion of the active regionfurthest from the peak is reasonably close to the peak. In a device withthree quantum wells separated by two barrier layers, the center of themiddle quantum well is optimally located at a distance from thep-electrode corresponding to a peak on a curve of extraction efficiencyvs. distance.

In the embodiment illustrated in FIGS. 6A and 6B, the total thickness ofthe active region is limited such that all of the active region is closeto the peak on the curve of extraction efficiency vs. distance betweenthe active region and the p-electrode. For example, the total thicknessof the active region may be selected such that the active region may notbe larger than 0.35λ_(n) and is usually not larger than 0.15λ_(n), andusually does not extend beyond 0.05λ_(n), on either side of the peak.The four curves in FIG. 4 demonstrate that as the active region becomesmore compact, the total extraction efficiency improves. FIG. 4illustrates the extraction efficiency of four devices, a single quantumwell device (curve a), a two quantum well device with a thin barrier(curve b), a two quantum well device with a thick barrier (curve c), anda four quantum well device. At the first peak shown in FIG. 4, thedevice with the most compact active region (the single quantum welldevice) has the highest extraction efficiency, while the device with thethickest active region (the four quantum well device) has the lowestextraction efficiency, assuming uniform filling of carriers in thequantum wells.

In embodiments with few (e.g. 1-3) quantum wells, the center ofbrightness of the active region is expected to be the physical center ofthe active region. In active regions with quantum wells of differentcomposition or thickness, or with more than three quantum wells, thecenter of brightness may not be the physical center of the activeregion. In such devices, the center of brightness of the active regionmay be located at a peak on a curve of extraction efficiency vs.distance.

In some embodiments, the quantum wells in the active region may beclustered around peaks on a curve of extraction efficiency vs. distance,with thin barrier layers separating the quantum wells in each clusterand thick barrier layers separating the clusters. The center ofbrightness of each cluster may be placed at a separation correspondingto a maximum on the appropriate plot of extraction efficiency oftop-side flux as a function of distance between the active region andthe reflective contact. FIGS. 7A and 7B illustrate such a device. Thedevice illustrated in FIG. 7A has two clusters of quantum wells, eachwith two quantum wells. The first cluster includes two quantum wells 15a separated by a barrier layer 17 a, the center of which is located atthe first peak shown in FIG. 4 (the second local maximum). The secondcluster includes two quantum wells 15 b separated by a barrier layer 17b, the center of which is located at the second peak shown in FIG. 4(the third local maximum). The two clusters are separated by a thickbarrier layer 17 c. The clusters may have more or fewer than two quantumwells, and need not have the same number of quantum wells.

FIG. 9 is an exploded view of a packaged light emitting device. Aheat-sinking slug 100 is placed into an insert-molded leadframe 106. Theinsert-molded leadframe 106 is, for example, a filled plastic materialmolded around a metal frame that provides an electrical path. Slug 100may include an optional reflector cup 102. Alternatively, slug 100 mayprovide a pedestal without a reflector cup. The light emitting devicedie 104, which may be any of the devices described above, is mounteddirectly or indirectly via a thermally conducting submount 103 to slug100. An optical lens 108 may be added.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. For example, the invention is not limited tothe contact materials and semiconductor materials described in theexamples. Specifically, though many of the examples are directed toIII-nitride flip chip devices with silver contacts, the invention mayalso be applicable to other reflective contacts and to other materialssystems, such as III-phosphide, III-arsenide, and II-VI materials.Therefore, it is not intended that the scope of the invention be limitedto the specific embodiments illustrated and described.

1. A light emitting device comprising: a region of first conductivitytype; a region of second conductivity type; an active region disposedbetween the region of first conductivity type and the region of secondconductivity type, the active region being capable of emitting lighthaving a wavelength λ_(n) in the region of second conductivity type; anda surface reflective of the light emitted by the active region, whereinone of the region of first conductivity type and the region of secondconductivity type is disposed between the active region and thereflective surface; wherein the active region has a total thickness lessthan or equal to about 580 angstroms and wherein a portion of the activeregion is located between 1.1λ_(n) and 1.4λ_(n) from the reflectivesurface.
 2. The light emitting device of claim 1 wherein the activeregion comprises three quantum well layers separated by two barrierlayers.
 3. The light emitting device of claim 2 wherein each of thequantum well layers has a thickness ranging between about 10 and about60 angstroms, and wherein the barrier layer has a thickness ranging fromabout 50 to about 200 angstroms.
 4. The light emitting device of claim 1wherein the active region has a thickness less than about 320 angstroms.5. The light emitting device of claim 4 wherein the active regioncomprises two quantum well layers separated by a barrier layer.
 6. Thelight emitting device of claim 1 wherein the active region comprises asingle quantum well layer.
 7. The light emitting device of claim 1wherein the active region comprises at least one III-nitride layer. 8.The light emitting device of claim 1 wherein the active region comprisesat least one III-phosphide layer.
 9. The light emitting device of claim1 wherein the surface comprises silver.
 10. The light emitting device ofclaim 1 wherein the surface comprises gold.
 11. The light emittingdevice of claim 1 wherein the surface comprises aluminum.
 12. The lightemitting device of claim 1 wherein a portion of the active region islocated between 1.2λ_(n) and 1.35λ_(n) from the reflective surface. 13.The light emitting device of claim 1 wherein a physical center of theactive region is located at a distance from the reflective surfacecorresponding to within 0.05λ_(n) from a local maximum in extractionefficiency.
 14. The light emitting device of claim 1 wherein a center ofbrightness of the active region is located at a distance from thereflective surface corresponding to within 0.05λ_(n), from a localmaximum in extraction efficiency.
 15. A light emitting devicecomprising: a region of first conductivity type; a region of secondconductivity type; an active region disposed between the region of firstconductivity type and the region of second conductivity type, the activeregion being capable of emitting light having a wavelength λ_(n) in theregion of second conductivity type; and a surface reflective of thelight emitted by the active region, wherein one of the region of firstconductivity type and the region of second conductivity type is disposedbetween the active region and the reflective surface; wherein the activeregion has a total thickness less than or equal to about 580 angstromsand wherein a portion of the active region is located between 0.1λ_(n)and 0.3λ_(n) from the reflective surface.
 16. The light emitting deviceof claim 15 wherein the active region comprises three quantum welllayers separated by two barrier layers.
 17. The light emitting device ofclaim 15 wherein each of the quantum well layers has a thickness rangingbetween about 10 and about 60 angstroms, and wherein the barrier layerhas a thickness ranging from about 50 to about 200 angstroms.
 18. Thelight emitting device of claim 15 wherein the active region has athickness less than about 320 angstroms.
 19. The light emitting deviceof claim 18 wherein the active region comprises two quantum well layersseparated by a barrier layer.
 20. The light emitting device of claim 15wherein the active region comprises a single quantum well layer.
 21. Thelight emitting device of claim 15 wherein the active region comprises atleast one III-nitride layer.
 22. The light emitting device of claim 15wherein the active region comprises at least one III-phosphide layer.23. The light emitting device of claim 15 wherein the surface comprisessilver.
 24. The light emitting device of claim 15 wherein the surfacecomprises gold.
 25. The light emitting device of claim 15 wherein thesurface comprises aluminum.
 26. The light emitting device of claim 15wherein a physical center of the active region is located at a distancefrom the reflective surface corresponding to within 0.05λ_(n) from alocal maximum in extraction efficiency.
 27. The light emitting device ofclaim 15 wherein a center of brightness of the active region is locatedat a distance from the reflective surface corresponding to within0.05λ_(n) from a local maximum in extraction efficiency.
 28. A lightemitting device comprising: a region of first conductivity type; aregion of second conductivity type; an active region disposed betweenthe region of first conductivity type and the region of secondconductivity type, the active region being capable of emitting lighthaving a wavelength λ_(n) in the region of second conductivity type; anda surface reflective of the light emitted by the active region, whereinone of the region of first conductivity type and the region of secondconductivity type is disposed between the active region and thereflective surface; wherein the active region has a total thickness lessthan or equal to about 580 angstroms and wherein a portion of the activeregion is located between 0.6λ_(n) and 0.75λ_(n) from the reflectivesurface.
 29. The light emitting device of claim 28 wherein the activeregion has a thickness less than about 320 angstroms.
 30. The lightemitting device of claim 28 wherein the active region comprises a singlequantum well layer.